1. Field of the Invention
Disclosed herein is a high frequency amplifier. More particularly, disclosed herein is a dual band high frequency amplifier using a composite right/left handed (CRLH) transmission line (TL), in which an input matching circuit and/or an output matching circuit in the high frequency amplifier are formed using the CRLH TL, thereby obtaining both gain and matching characteristics at two frequencies.
2. Description of the Related Art
A transmission line (TL) that allows electromagnetic waves to be transmitted has a variety of physical forms. However, the TL is generally represented as an equivalent circuit, as shown in FIG. 1, which includes a serial inductor having inductance LR per unit length and a shunt capacitor having capacitance CR.
An ideal TL has an all-pass characteristic. However, a practical TL has a low-pass characteristic with a specific cutoff frequency because of the loss of the TL itself and the limitation of the maximum operating frequency due to parasitic components. When the values of elements and parasitic components in a unit equivalent circuit are very small, the cutoff frequency of the TL is considerably high. Therefore, the low-pass characteristic of the TL seems to be the all-pass characteristic in a frequency to be practically applied.
As shown in FIG. 1, an equivalent circuit that includes a serial inductor having inductance LR per unit length of a TL and a shunt capacitor having capacitance CR may be represented as a T-shaped symmetric equivalent circuit that includes two serial inductors each having inductance LR/2 per unit length of the TL and a shunt capacitor having capacitance CR.
The TL may have a structure in which the positions of the serial inductance and the shunt capacitor in the equivalent circuit shown in FIG. 1 are switched to each other, i.e., a TL structure represented by a serial capacitor having capacitance CL and a shunt inductor having inductance LL as shown in FIG. 2. When compared with a general TL structure, such a TL structure has a characteristic that the propagating directions of group and phase velocities are opposite to each other. Such a TL is referred to as a left handed (LH) TL.
As shown in FIG. 2, the unit equivalent circuit of the LH TL, that includes a serial capacitor having capacitance CL and a shunt inductance having inductance LL, may be represented as a T-shaped symmetric equivalent circuit that includes two serial capacitors each having capacitance 2CL and a shunt inductor having inductance LL.
In order to be compared with the LH TL shown in FIG. 2, the general TL represented as the equivalent circuit shown in FIG. 1 is referred to as a right handed (RH) TL. A general RH TLs include a single- or multi-layer microstrip TL, a stripline TL, a coplanar waveguide (CPW) TL, a coplanar strip (CPS) TL, and so on.
Meanwhile, there exists a TL structure in which the structures of LH and RH TLs are mixed together. Such TL structure is referred to as a composite right/left handed (CRLH) TL structure (C. Caloz, C. Allen, and T. Itoh, “Unusual Propagation Characteristics in CRLH Structures,” in Proc. IEEE AP-S International Symposium, pp. 3549-3552, Monterey, Calif., June 2004). The CRLH TL structure is generally configured so that LH and RH TL structures are intentionally mixed together. However, in the practical application of an LH TL structure, the physical basis for interconnection is an RH TL structure. Therefore, if the LH TL structure is connected to or inserted into the RH TL structure, a CRLH TL structure can be consequently formed without making special efforts to implement the RH TL structure.
Such a CRLH TL structure basically has dual band characteristics originated from phase characteristics of the RH and LH TL structures. Thus, the CRLH TL structure can be used as a TL that requires a dual band, and research papers for the CRLH TL structure have been reported recently.
However, the dual band properties of the CRLH TL are not applied to high frequency active circuits such as amplifiers but limited to only passive circuits. This is because the passive circuits are mostly implemented with TLs, and if CRLH TLs are used in the passive circuits, passive circuits having dual band characteristics can be designed directly.
So far, no dual band amplifier using a CRLH TL has been reported.